Article

# SrxBa1−xNb2O6−δ Ferroelectric-Thermoelectrics: Crystal Anisotropy, Conduction Mechanism, and Power Factor

(Impact Factor: 3.3). 01/2010; 96(3):031910. DOI: 10.1063/1.3291563

ABSTRACT

Nonstoichiometric tungsten bronze-structured ferroelectric SrxBa1-xNb2O6-d (SBN) single crystals were found to be a promising n-type thermoelectric oxide. Thermopower anomalies were observed at the phase transition temperatures, depending on the degree of reduction as well as crystal anisotropy. Above 500 K, heavily reduced SBN crystals show high thermoelectric power factors (~20 W/cm K2 at 516 K) with both thermopower and electrical conductivity higher parallel to the c-axis. It is noted that the power factor increases with temperature due to the semiconducting behavior with high carrier concentration. The carrier transport mechanism also varies with the degree of reduction and temperature.

### Full-text

Available from: Soonil Lee,
• ##### Article: The Electrical Conductivity of Strontium-Barium Niobate
[Hide abstract]
ABSTRACT: We propose an explanation for the high electrical conductivity of the ferroelectric strontium-barium niobate. As the temperature T approaches the ferroelectric transition T c, the static dielectric constant $\varepsilon(0)$ diverges when a soft mode occurs. This divergence of $\varepsilon(0)$ reduces the donor binding energy, and increases the effective Bohr radius of the donor. The electrons bound to the donors become unbound, and the material becomes conductive.
Journal of Electronic Materials 07/2012; 42(7). DOI:10.1007/s11664-012-2248-6 · 1.80 Impact Factor
• Source
##### Article: Thermoelectric Power Factor Enhancement of Textured Ferroelectric SrxBa1−x Nb2O6−δ Ceramics
[Hide abstract]
ABSTRACT: A promising n-type thermoelectric oxide, based on the tungsten bronze-structured ferroelectric SrxBa1–xNb2O6–δ (SBN, x~0.61), was investigated to enhance the thermoelectric power factor through templated grain growth (textured polycrystalline). In the reduced SBN textured, both the electrical conductivity (σ) and the magnitude of thermopower (S) are increased in the c axis: σ33 > σ11 and |S33| > |S11|, and consequently, the thermoelectric power factor (PF) increased significantly due to crystal anisotropy and grain boundary density reduction. It was found in randomly oriented polycrystalline ceramics that the thermoelectric properties are dominated by a-axis properties. A ferroelectric–thermoelectric anomaly is observed at 4mm–4/mmm phase transition temperature (TC) and depends on temperature and reduction degree, consistent with our earlier observations in single crystal SBN. Above TC, the carrier transport mechanism is controlled by polaron hopping conduction, and below TC the behavior depends on the degree of reduction. However, the magnitude of the Seebeck coefficient is dependent on the crystal anisotropy.
01/2011; 26(01):26 - 30. DOI:10.1557/jmr.2010.78
• ##### Article: Crystal Structure and Dielectric Properties of Sr5RTi3Nb7O30 (R=La, Nd, Sm, and Eu) Tungsten Bronze Ceramics
[Hide abstract]
ABSTRACT: Effects of the radius difference (ΔR) between A1 and A2 site cation on the crystal structure and the dielectric properties were investigated for Sr5RTi3Nb7O30 (R=La, Nd, Sm, and Eu) tungsten bronze. The tetragonal tungsten bronze structure was determined for all compounds, and the octahedral distortion significantly varied with the composition. In the Ti/Nb(2)O6 octahedra, besides the off center displacement of the Ti/Nb(2) cations along the c-axis, the four oxygen ions in the equatorial plane were gradually driven to different planes with decreasing R cation radius, while displacement of the Ti/Nb(2) cations in the ab plane decreased. For the Ti/Nb(1)O6 octahedra, the equatorial plane deviated from the center of the octahedron when the R cation radius became smaller. The ferroelectric transition in Sr5RTi3Nb7O30 was also dominated by the radius difference between A1 and A2 site cation. When the R cation radius became smaller, the dielectric anomaly of Sr5RTi3Nb7O30 changed from relaxor-like behavior into first-order ferroelectric transition. Structural analysis indicated that compounds with larger radius difference between A1 and A2 site cation exhibited severer octahedra distortion, which was helpful for the ferroelectric transition. The low temperature relaxation observed in Sr5RTi3Nb7O30 (R=Nd, Sm, and Eu) was attributed to the freeze-out of polarizability in the ab plane, which originated from the Ti/Nb(2) cation displacement in the ab plane of the Ti/Nb(2)O6 octahedra.
Journal of the American Ceramic Society 02/2011; 94(6):1829 - 1836. DOI:10.1111/j.1551-2916.2010.04327.x · 2.61 Impact Factor